1. Field of the Invention
The present invention relates to a process and to a circuit arrangement for the control of the current through the primary winding of an ignition coil for coil ignition systems in motor vehicles, which current has a character that is largely independent of engine speed, supply voltage and the internal resistance of the ignition coil.
2. Description of the Prior Art
The essential feature for the spark power of a motor vehicle ignition system is the magnitude of the primary current in the ignition coil, and the second feature is the peak current value which flows at the moment of initiation of arcing. In conventional ignition systems, the flow of current is directed through the primary coil directly through a breaker contact (points). During the closing time of the breaker contact, a current, rising according to an exponential function, flows through the primary winding of the ignition coil, whereby a magnetic field is built up which generates the high voltage in the secondary winding of the ignition coil during the open time of the contact, which voltage is necessary for the ignition of the gasoline-air mixture in the cylinder. The speed of the rise of the primary current through the ignition coil is determined by the relationship of inductance to the ohmic resistance of the coil. The closing angle of the contact is selected to be large enough that the primary current can rise, as much as is possible, up to the optimum final point, even in the case of the highest revolutions per minute. For this reason, one aims for the largest possible closing angle.
On the other hand, in the case of low revolutions per minute, it must be provided that the primary current does not rise to impermissibly high values during the long closing time of the breaker contact. For this reason, the smallest possible closing angle is desired. Since, however, for the reasons cited above, the closing angle cannot be selected as small as may be desired, a resistance that limits the current is usually connected in series with the primary winding of the ignition coil. In ignition systems common on the market, the closing angle of the breaker contact must be adjusted to a compromise value.
A further disadvantage of conventional ignition systems which are controlled directly through the points is that in case of motor idle, a constant current can flow through the primary winding of the ignition coil when the points happen to be closed. In such a situation, the battery is not only unnecessarily discharged, but the primary winding is also impermissibly loaded. For example, it can be seen from the German published application 24 48 915 a circuit arrangement which interrupts the flow of current through the primary winding when no ignition pulses are generated for a longer time. To this end, a capacitor is provided in this known circuit, whose potential is proportional to the number of ignition pulses occurring per unit of time. When ignition pulses are absent, the capacitor discharges and blocks the flow of current through the primary winding of the ignition coil by means of a transistor. The disadvantage in the case of this circuit, however, is that in the starting process a plurality of ignition pulses must first be generated by turning the starter until the capacitor is charged to the point that the transistor releases the flow of current through the primary winding.